There are several processes for producing Vitamin C. One process involves a number of chemical synthesis steps and one fermentation step. Briefly, the steps are hydrogenation of glucose to sorbitol, fermentation of sorbitol to sorbose using Acetobacter suboxydans, sorbose acetonization, diacetone sorbose oxidation to 2-KLG, esterification of 2-KLG, and conversion of the ester to ascorbic acid. This process is complex and requires a relatively high capital investment for an operating plant.
Another process involves two fermentation steps. The process starts with fermentation of glucose to 2,5-diketo-D-gluconate (2,5-DKG) by Erwinia sp.; fermentation of 2,5-DKG to 2-KLG by Corynebacterium sp.; esterification of 2-KLG; and conversion of the ester to ascorbic acid. One study has shown that D-gluconate and 2-keto-D-gluconate (2-KDG) are produced sequentially from glucose by Erwinia sp. before 2,5-DKG is produced in the first fermentation step. See T. Sonoyama et al., "Production of 2-keto-L-gulonic acid from D-glucose by Two-Stage Fermentation," App. and Envir. Microbiol., 43, 1064-69 (1982). This two-step fermentation process, although having a somewhat lower capital cost than the Acetobacter process, is still complex and expensive to operate.
Still another process for converting glucose to 2-KLG is referred to in European patent application No. 132,308. That application refers to the conversion of glucose to 2-KLG in a single step fermentation process. It first refers to Corynebacterium sp. ATCC 31090 as a source of a DNA sequence coding for a particular 2,5-DKG reductase (an enzyme that is said to catalyze the fermentation of 2,5-DKG to 2-KLG). This DNA sequence, with its own or a synthetic ribosome binding site, is then said to be inserted "downstream" of an E. coli trp or tac promoter or the pACYC184 CAT promoter in an expression vector. The vector is also said to contain a gene coding for tetracycline resistance or other selectable marker, and an origin of replication derived from plasmids ColE1, 115A, or RSF 1010. A host cell, Erwinia herbicola (ATCC 21998), is then said to be transformed with the vector. On fermentation this transformed cell is said to produce 2-KLG from glucose in one step. The conversion of glucose to 2-KLG in that process, however, is not fully satisfactory because the yield of 2-KLG is very low and the time of fermentation to obtain even that low yield is too long.
Accordingly, a single organism capable of converting a carbon source, such as glucose, to 2-KLG at acceptable rates and in a single fermentation step is still a goal that has not been attained.